Fullerenes and nanotubes are the most desirable materials for basic research in both chemistry and physics, as well as research in electronics, non-linear optics, chemical technologies, medicine, and others.
The processes of producing new allotrope forms of carbon, fullerenes, nanotubes and nanoparticles (buckyonions) are based on the generation of a cool plasma of carbon clusters by an ablation of carbon-containing substances, driven by lasers, ion or electron beams, a pyrolysis of hydrocarbons, an electric arc discharge, resistive or inductive heating, etc., and clusters' crystallization to the allotropes under certain conditions of annealing (R. E. Smalley, From Balls to Tubes to Ropes: New Materials from Carbon, in Proc. of American Institute of Chemical Engineers, South Texas Section, January Meeting in Houston, Jan. 4, 1996), after which fullerenes are usually eluted from the soot by the use of aromatic solvents, such as benzene, toluene, xylenes, chlorobenzene, 1,2-dichlorobenzene, and the like (P. M. Ajayan et al., Nature, 1993, v.362, p.522). Nanotubes on the other hand are separated from soot and buckyonions by the use of gaseous (air, oxygen, carbon oxides, water steam, etc.) (Ebessen et al., U.S. Pat. No. 5,641,466, Issued Jun. 24, 1997, Method of Purifying Carbon Nanotubes) or liquid oxidants (nitric, hydrochloric, sulfuric and other acids or their mixtures) (Andreas Thess et al., Science, 273, 483-487, Jul. 26, 1996).
The processes of forming different carbon allotropes (for instance, fullerenes and nanotubes/buckyonions) are competitive and, therefore, it is possible to displace the balance in their output by changing conditions either of the generation process or of crystallization (annealing). In arc discharge processes, increasing the pressure of a buffer gas (He or Ar) from 50-150 Torr, which is optimal for producing fullerenes, to 500 Torr leads to a preferential formation of Multi-Wall Nano Tubes (MWNT)/onions (K. S. Khemani et al., J. Org. Chem., 1992, v.57, p.3254). Addition of some metal catalyzers (Co, Ni, Pt, Fe, etc.) to the initial graphite donor leads to preferential formation of Single-Wall NanoTubes (SWNT) (W. Kraechmer et al., Nature, 1990, v.347, p.354) with a yield up to 70% in case of a laser ablation of said graphite.
Several methods are used for producing SWNTs (Andreas Thess et al., Science, 273, 483-487, Jul, 26, 1996), each of which differs in the generation of the cool carbon clusters' plasma. Comparatively low outputs of SWNTs lead to very high prices of the SWNTs ($1,000-10,000 per gram).
The process of producing higher fullerenes (the fullerenes higher than C70) is developed to a less extent that the process for the classical production of C60 and C70.
The main problem is a very low yield of the higher fullerenes which for C74, C76, C78, and C84 is usually about 1-3% and less than 0.1% for C90, C94, C98 in comparison to the 10-40% for the classical fullerenes (K. S. Khemani et al., J. Org. Chem., 1992, v.57, p.3254). As a result, the amounts of C90 and higher available are not enough to study their general properties.
Thus, a method and device are required for producing higher fullerenes and carbon nanotubes with an attainable result (greater amounts of higher fullerenes and nanotubes at a relative low cost), that is expressed as preferably producing the higher fullerene and in a simple separation of the nanotubes from the soot particles.
The existing method and device for producing fullerenes (W. Kraechmer et al., Nature, 1990, v.347, p.354) suggests the following. That graphite electrodes should be placed in a contained volume filled by He gas at a pressure of 50-150 Torr. Under certain conditions (electric arc's current is up to 200 A and voltage in the range 5-20 V) evaporated graphite clusters can form fullerene molecules, mainly C60 (80-90%) and C70 (˜10-15%) as well as small amounts of higher fullerenes (total sum is not exceeding 3-4%). High Performance Liquid Chromatography (HPLC) is required to separate individual fullerenes (F. Diederich et al., Science, 1991, v.252, p.548).
HPLC is characterized by a very low production of higher fullerenes and as a result market prices of the higher fullerenes are enormous, more than $1,000-£10,000 per gram (Alderich catalog, 1999). Therefore this method and device is useless for producing higher fullerenes. Outputs of C76, 78, 84 for such technology is about a couple of milligrams a day per processor, even less for higher fullerines.
The use of both relatively low arc currents and special metallic catalysts are needed for producing single-walled carbon nanotubes with certain diameters. The maximal nanotube output achieved is 60% of the graphite material scraped from a cathode surface. The total nanotube output is greatly decreased during the separation of the nanotubes from the rest of the soot particles when an oxidation process with gases (oxygen, carbon dioxide) is usually used. Moreover, the separation process is rather long and complicated.
It is therefore necessary to find an approach which allows production of higher fullerenes and nanotubes with higher yields.
For the C74 fullerene such a way has been realized (F. Diederich et al., Science, 1991, v.252, p.548) by the use of a constant current arc-discharge in a liquid benzene and/or toluene medium, which dissolves fullerenes well. The dominant fullerene molecules were C50, whereas the concentration of C60 and C74 was comparable but 3-8 times less than that of C50. All fullerenes produced were dissolved in the medium and, after removal of non-dissolved soot particles (either by centrifugation or filtration) fullerenes could be separated by HPLC.
However, no fullerenes greater than C74 or SWNTs were produced this way. The greatest problem of all the methods is the use of an electric arc discharge that provides a gap of constant value between the graphite rods. In observing Modak's method (D. K. Modak et al., Indian J. Phys., 1993, v.A67, p.307) a safety problem arose because of the release of huge amounts of gases (mainly, hydrogen and acetylene) in the process of cracking benzene/toluene.
The basic method for producing MWNT/buckyonions (K. S. Khemani et al., J. Org. Chem., 1992, v.57, p.3254) using a DC arc discharge of 18V voltage between a 6 mm diameter graphite rod (anode) and a 9 mm diameter graphite rod (cathode) which are coaxially disposed in a reaction vessel maintained in an inert (helium at pressure up to 500-700 Torr) gas atmosphere has a problem because it is not possible to continuously produce carbon nanotube/buckyonion deposit in a large amount because the deposit is accumulated on the cathode as the anode is consumed. It is required to maintain a proper distance (gap) between the electrodes.
Oshima et al. (U.S. Pat. No. 5,482,601, Issued Jan. 9, 1996, Method and Device for the Production of Carbon Nanotubes) suggest a complicated mechanism for maintaining the gap (preferably in the range from 0.5 to 2 mm) between the electrodes at the same DC voltage (preferably 18-21 V)/current (100-200 Amp) and for scraping the cathode deposit during the process. As a result, they are able to produce up to 1 gram of a carbonaceous deposit per hour per one apparatus (pair of electrodes). A nanotube/buckyonion composition of the deposit is supposed to be the same as in (T. Ebessen et al., Nature, 358, 220, 1992, or T. Guo et al., Chem Phys. Lett., 1995, v.243, p.49), i.e., nanotube: carbon nanoparticles (buckyonions) 2:1. A specific consumption of electric energy is about 2-3 kW·hour per one gram of the deposit. Complexity of the device, high specific energy consumption plus consumption of the expensive inert gas, helium, are the most factors that restrain bulk production of MWNT/buckyonion deposits by this method.
Instead of these methods, to produce nanotubes in bulk Olk (U.S. Pat. No. 5,753,088, Issued May 19, 1998, Method for Making Carbon Nanotubes) suggests simplifying a DC arc discharge device by immersing carbonaceous electrodes in a liquefied gas (N2, H2, He, Ar or the like). The other arc parameters are nearly the same (1 8V-voltage, 80 Amps-current, 1 mm-gap, 4-6 mm in diameters-electrodes). However, such a “simplification” leads even to poorer results than those in the methods mentioned above. It was possible to maintain an arc between the electrodes just for 10 seconds, therefore, the production was very low. A composition of the deposit was nearly the same as in the previous ones.
To improve properties of the said deposits they suggest purifying and uncapping MWNTs (Ebessen et al., U.S. Pat. No. 5,641,466, Issued Jun. 24, 1997, Method of Purifying Carbon Nanotubes; Andreas Thess et al., Science, 273, 483-487, Jul. 26, 1996) by using gaseous/liquid oxidants and filling the uncapped nanotubes with different materials (metals, semiconductors, etc.) to produce nanowires/nanodevices. Tips of nanotubes are more reactive than side walls of buckyonions. As a result, oxidation only carbon nanotubes are finally left while buckyonions disappear.
Recently, it has been discovered that buckyonions are very promising material to produce diamonds. However, known processes produce less buckyonions than nanotubes and purifying of deposit by using known methods leads to a complete reduction of buckyonions. Therefore, it is required to find an improved process for producing or purifying buckyonions.
It is required to uncap nanotubes to fill them with metals (to produce nanowires) or other substances, like hydrogen (to create a fuel cell).
The main problem for uncapping the tubes by known methods is supposed to be that under the oxidation the tube ends become filled with carbonaceous/metallic debris that complicates filling the open-ended tubes with other materials after oxidation, finally reducing an output of the filled nanotubes.
Chang (U.S. Pat. No. 5,916,642, Issued Jun. 29, 1999, Method of Encapsulating a Material in a Carbon Nanotube) suggests a method of encapsulating a material in a carbon nanotube in-situ by using a hydrogen DC arc discharge between graphite anode filled with the material and graphite cathode. The main difference from the above mentioned inventions is the use of hydrogen atmosphere to provide conditions for encapsulating the material inside nanotubes during the arc-discharge, i.e., in-situ. All the arc discharge parameters are nearly the same as in the above mentioned inventions (20V-voltage, 100 Amp-current, 150 A/cm2-current density, 0.25-2 mm-gap, 100-500 Torr-pressure of the gas). The presence of hydrogen is thought to serve to terminate the dangling carbon bonds of the sub-micron graphite sheets, allowing them to wrap the filling materials. Judging by TEM examination of the samples produced by this method, about 20-30% of nanotubes with diameters of approximately 10 nm are filled with copper. The range of germanium filled nanotubes is 10-50 nm and their output is much lower than that of the copper filled nanotubes. A use of helium atmosphere (at the same pressure in the range of 100-500 Torr) instead of hydrogen leads to a preferable formation of fullerenes, cooper or germanium nanoparticles and amorphous carbon (soot particles) with no nanotubes at all. A mixture of hydrogen and an inert (He) gas may be used for the encapsulation as well.
Shi et al. (Z. Shi et al., Mass Production of SWNT by Arc Discharge Method, Carbon, v.37, n.9, pp.1449-1453, 1999) have reported a mass production of SWNTs by a DC arc discharge method with a Y—Ni alloy composite graphite rod as anode. A cloth-like soot is produced, containing about 40% SWNTs with diameter about 1.3 nm. The most important feature of this invention is the addition of Y—Ni alloy in the anode. However, the yield of the deposits and specific energy consumption are nearly the same as in the inventions described above. Other References include Hiura et al., U.S. Pat. No. 5,698,175, issued Dec. 16, 1997, Process for Purifying, Uncapping and Chemically Modifying Carbon Nanotubes; R. O. Loutfy, rloutfy@mercorp.com; N. Sivarman et al., J. Org. Chem., 1992, v.57, p.6007; M. T. Beck, G. Mandi, Fullerene Sci. Technol., 1996, v.3, p.32; X. Zhou et al., Fullerene Sci. Technol., 1997, v.5(1), p.285; V. A. Ryzhkov, in Abstracts of Intern. Workshop on Fullerenes and Allotropes of Carbon, IWFAC '99, 3-8 Oct. 1999, St. Petersburg, Russia; A. P. II'in, Yu. A. Kracnyatov, G. A. Volostnov, Y. T. Galeev, ICI C01 B31/00, The Device and Method for Producing Fullerenes—Application to a Russian Patent (Tomsk High Voltage Institute, Priority from September 1997); and R. S. Ruoff et al., J. Phys. Chem., 1993, v.9, P.3379.
A major drawback to these prior art processes is the low quantity of non-classical fullerenes, nanotubes and buckyonions produced. Typical production rates under the best of circumstances using these processes amount to no more than 1 g/h of a carbonaceous deposit containing 20-60% of nanotubes and 6-20% of buckyonions. Furthermore, the prior art processes are not easily scaled-up to commercially practical systems.